open bridgedeck catamaran design

Discussion in 'Multihulls' started by BobH, Apr 17, 2015.

  1. BobH
    Joined: Apr 2015
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    BobH Junior Member

    Looking for commentary/critique (aside from, "don't design it yourself"). I have put together a design for a catamaran to meet my list of requirements:

    1. catamaran, max 34 ft. LOA, 21 ft. beam

    2. good bridgedeck clearance…32 inch minimum

    3. minimum open bridgedeck, well back from bow and stern

    4. center of gravity at or below bridgedeck, with all tankage, engines, food storage near waterline

    5. epoxy-cedar strip hull bottom/ply on frame above (as in Woods Sagita/Eclipse)

    6. inboard diesels, kick-up drive legs, good fuel capacity (motorsailer…modified Sillette Sonic-sideways mounted engine in each hull-no universal joint)

    7. easy-to-handle, easy to balance sailplan...Ketch - main: vertical roller reefing (as in the wishbone gaff rig by Bernd Kohler), freestanding mizzen: sleeved sail with wishbone boom and gaff, jib: roller reefing

    8. balanced rudders

    9. daggerboards

    10. accommodations for four: small galley, drop-down table, enclosed head, two large double berths

    11. capable of offshore runs from Alaska to Chile and Hawaii, but mostly summertime sailing in NW Pacific coastal waters


    I am looking for some helpful observations/advice. I would like to do as much of the legwork of layout and design myself, including the basics of sail CE vs. CLR, distribution of weight, etc., but I do plan to engage a qualified naval architect to finalize the structure. For what it's worth: I enjoy designing what I build.
     

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  2. Petros
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    Petros Senior Member

    That looks like a very nice design, well thought out. I love the way you integrated the central salon with the hull berths.

    My personal preference would be on a 34 footer to use a sloop rig, less rigging and stuff to mess with and maintain. And I would also go with a plumb or vertical bow, it will have slightly less tendancy to dive and might give you a dryer ride than the racked back bow.

    To my eye the rudder sees a bit smallish, though that is without knowing any of the areas or ratios.

    Good luck.
     
  3. snowbirder

    snowbirder Previous Member

    I'd prefer a sloop on a boat of this type/size as well.

    Also... if your boards are up, those rudders hit the ground first.

    They need to be able to deal with hitting the ground and not breaking. Kickup or something.
     
  4. waikikin
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    waikikin Senior Member

    It's a common approach that rudder set up, in waters that run that risk you just drop the boards for a few inches of margin, of course mistakes do happen!

    Looking at the pics BobH might be including lifting blades in barrel/drum rudder configureation.

    I'd go for the sloop too.

    Jeff.
     
  5. John Perry
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    John Perry Senior Member

    I find this interesting, particularly since about 3 years ago I got some way into drawing a cruising catamaran which shares many features with your design - roughly similar overall dimensions (mine is a little smaller) - very similar proposed construction method (strip plank hull bases, plywood elsewhere) - open bridgedeck but with provision for storage and a wheel with seated helm position - similar continuation of the in-hull accomodation into the bridgedeck - very similar arrangement of the main entrance hatches into the hulls. Differences are that I drew a sloop rig, although I did not get as far as finalising the details of this, also mine was intended to be primarily a sailing boat rather than a motor sailer and as such it would have outboard engines, not diesels.

    I think your design looks to be well thought out, here are a few comments:

    Are the lines that run from the windward hull to the outer ends of the wishbone gaffs intended to control sail twist? This is the way 'vangs' were used to control sail twist on the spritsail barges that operated on the Thames estuary in the UK in the early decades of the 20 century. If this is so, it is an interesting idea and one I have wondered about at times. Although most mainsails probably do twist more than the optimum amount, some twist is desirable to lower the center of effort in wind gusts and it can probably also help to reduce induced drag. I suspect that over-restricting twist at the head of the sail would not be good. I also see a potential problem with combining the mizen gaff vangs with an unstayed mizen mast. The unstayed mast can be expected to bend away to leeward in gusts so if the outer end of the gaff is restrained by vangs a wind gust may well cause the angle of attack to be increased towards the top of the sail which I would have thought highly undesirable. With regard to the vangs on the main gaff, I wonder if a simpler way to control that gaff would be a line taken from a sheave at the top of the mizen to the outer end of the gaf, although I realise that would mean that the two masts would be tied together so that if one were to fail both probably would.

    I dont understand why you have a wishbone boom on the mizen but not on the main. Wishbone booms have been found to work well, so if you are going to have one on one sail why not on the other? Also, if you do want a boom on the mainsail, why does it need both a rigid kicking strap and a near semi-circular mainsheet track - I would have thought one or the other and if you go for just the mainsheet track you could still have a topping lift and possibly also lazy jacks to hold up the boom.

    I would have thought that you might want a roller foresail, maybe this is just not shown at this stage (my drawings also show very little detail of the rig).

    The deck area around the base of the main mast is sloped so may not be a good working area, but its no worse than on many other catamarans. Since you dont have accomodation in the area of the mast step I would have thought it would be possible to have at least a small flat working platform to handle halyards etc. at the base of the mast.

    Your proposed engine arrangement sounds very good for a motor sailer - if the outdrives are steerable you could even make the boat crab sideways into a parking space without the need for any bow or stern thrusters. However, it does sound like an expensive solution requiring quite a lot of custom engineering. I imagine that you will need to have bevel gears between the engine and the input to the outdrive leg, perhaps these would be fitted into streamlined 'blisters' on the inner topsides. The picture attached shows a possible alternative way to drive a retractable propeller from a transverse mounted engine - obviously an old idea! If the bevel gears are fairly near the waterline on the outside of the hull would not need nearly such a long shaft and the perhaps the shaft could swing down on a pivoting bracket. Just a thought.

    This being a motor-sailer I am surprised that you do not want the helm position fully sheltered, at least from ahead. A fold down windscreen/spray dodger could achieve this if it were not for the curved mainsheet track that would be in the way of it.

    It looks like you are intending to place the topside stringers on the outside of the boat. Certainly an interesting idea - should avoid the stringers taking up space in the accommodation and they should provide some protection for the topsides, sounds good.

    The drum mounted rudders should be feasible. At one stage in my rather varied career I worked for a small company that made specialist hardware for racing yachts and sailing super yachts. One of our projects, albeit not one that I was closely involved with, was a rudder in a rotating drum as you have drawn. This was for a monohull race yacht, I think the designer was Julian Everett, he might even have some information about it. As far as I know it worked fine, but it was quite a job to make - I think we had big custom made ball races at the top and bottom of the drum, Torlon balls and I think the races were non-metallic, maybe turned from something like Tuffnell, I cant quite remember the details.

    I dont want to divert this thread, but for comparison I attach a drawing of my one - not even a finished design at this stage. I got this far with it then started to get ideas for a novel lifting hydrofoil arrangement which would probably be best tested first on a smaller boat, or perhaps I should do like Doug Lord and start with a remote controlled model.
     

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  6. BobH
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    BobH Junior Member

    Mr Perry,
    Apparently we do have some similar ideas regarding design features. Excellent drawings...what software are you using?

    Yes, those are vangs. I am experimenting with the idea of using a stiff freestanding mast with a vang with a controlled stretch device (like the rubber band links used in exercise machines for resistance in place of weights). The vang would have a slack loop in it held slack by the bands...at full stretch the rope takes over and the vang is full on. I plan on trying a scaled down version of this rig this summer, if I can get the rig and boat done (15' double-ended sail/row boat). Sort of an adjustable, self-resetting circuit breaker for dealing with gusts rather than a quick hand on the vang. The force applied can be adjusted during trials by adding/reducing resistance. I want to avoid having to rebuild a carbon mast because the first try was too bendy.

    Using a freestanding mast on a multihull is not suggested just because of the huge righting moment...so I thought I should have some safety feature as part of the design in place of the customary engineering safety factor arrived at by tripling the forces designed for. This boat will have a righting moment of approximately 64,000 lb ft. A mast strong enough to handle three times that would be too fat and too heavy. So I want sail controls that will dump wind in a controlled way...adjustable as experience with the boat dictates.

    Because the mizzen is freestanding I do not want them linked...I agree, it would be a good angle for main twist control to take a line from the mizzen top...but too much stress for the mizzen to bear at the partner. As it is the mast has to be able to lift the leeward hull with a force centered at the CE for that sail. As far as I can calculate, adding the possible force due to the main at the very top would require too much of the mizzen (if it is of reasonable diameter and weight of construction).

    The rig is an amalgam of a freestanding version of a Wharram cat rig with Bernd Kohler's vertical furling main...hopefully done well. First reef: douse mainsail...40% reduction, CE shifts very slightly aft and down, 2nd reef: roll in some foresail and first reef mizzen...and so on. Hopefully the rig has the reported good manners of cat-ketch rigs like on a Core Sound 20 or Presto 30.

    Right, spiral bevel gearbox...good friend machinist/mechanic is willing and able to put the drive together. We'll also be able to put together the box cooler and dry stack exhaust system. Regarding the dodger...the freestanding mizzen is able to rotate 360 degrees. I don't want to limit that.

    The topsides are to be sandwich construction with inner layer 4mm ply, 3/4 in. stringers with core between, finished in 4mm ply, with fir stringer doublers outside then glassed. Sounds complicated, but not really. I hope to build the panels into one piece and attach the inner stringers on the floor, then hang the whole side in one go (with lots of careful measurements and plenty of help). Fill the spaces between stringers, fair, then the outside panel with its stringers already attached providing stiffness to the floppy 4mm panel. We'll see.

    Rudders...you are right...in drums with plastic bearings. I want to distribute rudder forces over a much larger bearing surface and still have them retractable. Rudders built to break before the drums do...carry spares.
     
    Last edited: Apr 23, 2015
  7. brian eiland
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    brian eiland Senior Member

    Single-Masted Ketch consideration

    First, my compliments on the extent that you have gone to in this design project you have provided. Secondly, the illustrations are great, I wish I had that computer capability, but I'm just old school...:)

    Since a number of folks have addressed your rig choice and design, I thought I might as well also. In general I believe a 34 footer is slightly small for a ketch rig, but your presentation is VERY nice, and very workable. I'll first just suggest an alternative, before commenting on your particular design.

    I've also been a ketch fan as you might discern from this posting:
    Aft-mast Origination and Justification

    Sails in Combination

    I just re-read this posting I made back in 2002, and I find that I covered a lot of the bridges correctly even then.
    Single-masted Ketch
    So even while I like you rig presentation very well, I believe you could derive even better driving power from mine, and with only one mast mounted approximately where your mizzen is.

    BTW, I may soon be doing some model test on this rig as I may have a client for the big Gamefishing version in the near future.
    http://www.boatdesign.net/forums/sailboats/scale-model-testing-sailing-rigs-outdoors-52977.html#post731871

    Regards, Brian
     
  8. John Perry
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    John Perry Senior Member

    Thanks. I tend to think that asking a draughtsman what drawing software he is using is a bit like saying to an author "I liked your last book, tell me, did you write it with a fountain pen or a ball point?". But since you ask, this was Solidworks. The other drawing software I have tried in recent years is Inventor, that could produce similar results and I expect Catia and Simens NX also could. Maybe Rhino too, but I do find parametric capability very useful and from what I have read, I am not sure that Rhino has that to the same extent.

    The idea of using elastic elements to relieve sail forces in high winds is one of those ideas that is pretty obvious but doesn't seem to have been looked into much, perhaps there is a reason for that. Maybe sails, rigging and spars have enough elasticity that there is no need to introduce additional elasticity. And of course, with monohulls, which are still the great majority of sailing yachts, the heeling effect more or less does the job anyway. I think there was a monohull yacht for which the whole rig could lean to leeward to spill wind without heeling the hull so much but the idea did not catch on. If you are going to experiment with elastic elements in the rigging I wonder if it would be better to have something that is stiff up to a certain load then becomes elastic, rather than a spring in parallel with a slack rope which is elastic first then becomes stiff. I don't know about the rubber bands used in gyms, not having been in a gym since my schooldays. Are they something like the rubber bands used for bungy jumping? Or those used for launching gliders? If you wanted a more 'engineered' option perhaps you could use a hydraulic ram connected to an accumulator with gas under pressure, perhaps tucked away below deck with a cable through a deck sheave. Or a big gas spring which amounts to the same thing. On a project I was involved with we didnt have the pressurised gas, just relied on the slight compressibility of hydraulic fluid together with the slight elasticity of a steel cylinder - it did work but not sure I would particularly recommend it - very high pressure! But rubber bands are probably simpler, or maybe just nylon rope, although maybe that creeps too much?

    Even with elasticity in the vangs, I think you will need to adjust the vangs as well as the sheets when you luff up or bear away, or do you have some clever scheme to make the vangs adjust automatically as the sheets are adjusted? The vangs on the Thames spritsail barges were adjusted independently of the sheets. The crew of skipper and mate would have to work mainsheet, vangs and foresail sheets as well as the leeboard winches when making a tack, but they managed it so it must be possible. I once had a day out on one of those barges and even though there was more crew than a working barge would have had, they left the leeboards to look after themselves when tacking because there was just so much else to do (like opening beer bottles).

    I dont think that the choice between a stayed or unstayed mast needs to be dependant on the overall size of the craft - I would guess that Rob Denny would happily take on the making of an unstayed mast for any size of yacht. In rough terms, if you increase the size of a yacht while keeping all the geometric measurements in the same proportions, the strength of an unstayed mast increases with the cube of linear dimensions as does the bending moment caused by wind loading (thats assuming that the bigger yacht will sail in the same wind strength as the smaller one, it also neglects windshear). The stability, i.e. maximum righting moment, does increase with the fourth power of linear dimensions, so if you make the yacht bigger you may like to scale up the rig a bit more than the other dimensions to take advantage of the 'surplus' stability available. But if you do that, I would have thought it reasonable to increase the diameter of an unstayed mast in proportion to the increase in the other linear dimensions of the rig, in which case, once again, strength and bending moment both increase with cube of linear dimensions, so all is well.

    With regard to the choice of stayed or unstayed masts, wide beam, as for multihulls in general, must surely tip the balance some way towards stays. That is not to say that you cannot use an unstayed mast on a multihull, or that unstayed masts on multihulls are necessarily a bad idea, more the case that if you think that stays are a good idea on a monohull then you will probably think that they are an even better idea on a multihull. Thinking of it that way, putting an unstayed mast on a multihull could seem a bit of a waste of an excellent shroud base.

    I am still wondering what will happen to the angle of attack towards the head of your mizzen sail when a wind gust comes. The top of the unstayed mast and hence the inner end of the gaff will surely move to leeward a bit, even if it is a stiff mast. You propose to have non linear elasticity in the vang that controls the outer end of the gaff, so anything is possible! One point is that the vangs will increase the bending moment on the mast in the region of the gaff inner end, presumably your outer forestay is intended to counteract that for the mainmast, but there is no extra support for the mizen mast. I had a quick Google for Bernd Kohler's vertical furling main but could find nothing. I guess that it must be something like a roller blind with the roller stretched down the back of the mast?

    You dont have the rudder drum axis perpendicular to the hull skin, so when the rudder turns you will have a step in the underside of the hull, presumably you know that and dont mind it.

    4mm ply sounds thin for the outer skin of such a large boat but the core (presumably some kind of foam) must help, even if it only carries compression load between the skins. I am not sure how easy it would be to get the core bonded to both plywood skins when those skins are also held apart by the stringers between them. Perhaps you are not even trying to do that, maybe the core is just a filler and thermal insulation rather than being intended to transfer shear load between the skins as in conventional foam sandwich. Looking closely, I see you have a rubbing strake on the inner hull topside, I would have thought that it is only the outer topside that may get bashed on quay sides.

    I wonder how it will be using anchors from the reverse raked bows. You have drawn extended anchor chain fairleads but even so, if the chain becomes vertical it looks like it might start chain sawing into the 4mm ply skin of the topsides, also maybe catch on the 'knuckle'. My drawings show an alternative position for the chain fairlead and anchor stowage, as in the fourth of my pics above. This is based on an assumption that it is a good idea to use a bridal to both bows when anchoring a catamaran, or alternatively you could have a chain hook on a line lead through a heavy duty block hanging under the forward cross beam. Or rather than a chain hook have a giant (probably custom made) snatchblock to carry the chain, then you could raise the anchor to the forward cross beam before bringing it back to the bridge deck. I saw a Dragonfly trimaran last time I went to a boat show and I noticed that it had the anchor stowage and chain fairlead on the main cross beam just outboard of the main hull, which is the trimaran equivalent of what I had drawn. The Dragonfly salesman said it worked really well.

    You don't seem to have very large windows in your boat. A matter of personnel preference of course, but I think I would like a good view out and to be able to see out while sitting at the table or even in bed. I would add that when I did an FEA on a somewhat simplified model of the structure I found that when the hulls were under torsion, for example when the boat is approaching a lee bow capsize, there were high stresses around the window openings and the structure in that area needed strengthening. The same was true of the deck around the outboard sides of the region which is cut away to allow access through sliding hatches into each hull.

    I hope you manage to get your smaller boat built so that you can do the experiments with your rig ideas, it all sounds interesting. The kind of thing that professional boat designers tend not to do because they don't have the time or they dont want to risk the effect on their reputation if the experiments dont work out well. But if an amateur does it and it happens to be the one in a thousand (or whatever) of the new ideas that actually does work, you can be sure that the professionals will be copying it in no time!

    Sorry if I ramble on somewhat

    ps, shouldn't this be in the multihulls section of the forum?
     
  9. BobH
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    BobH Junior Member

  10. BobH
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    BobH Junior Member

    Rudder drum

    "You dont have the rudder drum axis perpendicular to the hull skin, so when the rudder turns you will have a step in the underside of the hull, presumably you know that and dont mind it."

    Yes, that is a problem...my solution is to not make the bottom of the drum flat. Rather make it curved in cross-section close to the longitudinal cross-section of the hull bottom in that area...like a bowl bottom. So it presents a fill for the cut out for rudder motion +- 35 degrees
     

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  11. BobH
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    BobH Junior Member

    The core will be structural. I am working on the same principles shown in Gougeon's West System book...they often used both stringers and core, apparently (like on Rogue Wave's deck). For the outer layer I may even try laying down a flat fiberglass sheet on a melamine table on the floor in one piece, glue on the outer stringers, and then secure that sheet to the structure so I can visually inspect the bond through the glass (would have to be as thin as workable...probably would need to set up a low vacuum clamp on the whole side). Then, when it is set, overlay an appropriate layer of outer glass (right now I am thinking 40 oz total of uni and biax). So there would be only the inner layer of wood...the outer layer of FRP. I'll have to experiment with that first to see if it has a chance of working.

    The outer stringer is dual purpose...both protection from damage to the skin and to provide some stiffness in the FRP outer layer...like a hat section stiffener. So it is to be used on both sides of the hulls.
     
  12. BobH
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    BobH Junior Member

    Anchor lead

    The bow fittings are for an anchor bridle. The chain and anchor are stowed on a capstan at the rear of the bridgedeck. The idea is to deploy the anchor then bring the last bit of chain up to the bow to secure the bridle. A rope from the capstan to the bow does that last bit.
     
  13. BobH
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    BobH Junior Member

    Mr Perry,
    What tool do you have for an FEA analysis? I am new to this forum...I will try to link this thread to the multihull section....not sure how right now.
     
  14. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Fat-Headed, Squaretop Sails

    I know they are all the rage, but there are some questions about their use. I think there are some subject threads on this forum about that subject. Here is one:
    http://www.boatdesign.net/forums/sailboats/squaretop-mains-46803.html

    Most recently I added a question about which sail resulted in the capsize of the latest foiling Gunboat:
    In order to control the shape of your 2 fat-headed sails you are proposing to make use of 2 booms on each sail. It will work, but appears to be over complicated to what end?
     

  15. tspeer
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    tspeer Senior Member

    Some of the difficulties in using elastic elements are the large static forces required, the dynamic loads in a seaway, and the need to work equally well on both tacks.

    What you'd like to see is a very nonlinear response. The rig needs to be very stiff up to the limiting righting moment of the hull(s). Then it needs to yield rapidly to any increases beyond that. A linearly elastic rig will give too much at low loads, and not give enough at high loads.

    The rig has a high moment of inertia. This means the dynamic loads are going to be high as the boat pitches and rolls in a seaway. And gusts can result in significantly higher loads than the static operating load, but are of short duration so they don't have the impulse needed to capsize the boat. These factors add up to a lot of false alarm trips of something that acts like a mechanical fuse.

    The need to work equally well on both tacks means its hard to preload a linearly elastic rig with a low stiffness in order to achieve the nonlinear response. The preload would have to be released and then cranked back on every time the boat tacked. And there's a lot of energy associated with a high preload on an elastic structure. The softer the stiffness, the more energy that needs to be put in in order to wind up the structure to get to the static force.
     
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